Annoyingly, PHP errors like this are difficult to debug. The implication is that this is coming from the find_new_wrapper template, which doesn't have any avatar elements out of the box. Have you customized this?

Alternatively, it could be within find_new_posts (or an included template). It (unfortunately) would be something that we'd need to test with the default style and no add-ons as I don't think this is something we've seen before.

ErrorException: Fatal Error: Method XenForo_Template_Public::__toString() must not throw an exception, caught TypeError: Argument 1 passed to XenForo_Template_Helper_Core::helperAvatarHtml() must be of the type array, boolean given, called in /www///-/library/XenForo/Template/Helper/Core.php on line 853

The URL that triggered it would help, but it's likely to be a bad template customization. The URL would allow me to at least give you an idea of where to look, but you'd likely need to start editing relevant templates to track down the specific call that's causing problems.